It has been discussed previously (spam filters in general rather than intergration) - spam filters would probably be best kept away from being included with DirectAdmin - simply due to the fact an effective spam filter will almost always have false positives, maybe not for things such as webhosting although, as was said previously - contents of emails will vary dramatically depending on each and every individual user.
The best thing to do, that is safe, and useful is simply have a user definable spam filter that allows users for each domain to allow specific words filtered - if the email has the word then it is dropped or bounced.
I am considering writing an add-on for DA that allows spam filters such as spamassassin or RBLs to be setup and configured *very* simply and with minimal knowledge - how effective it would be is another question (providing it was done globally and affecting all users)
Chris